The present invention relates to a circuit arrangement for connecting user instruments at user, or subscriber, stations to a digital time multiplex communications network with decentralized exchange.
U.S. patent application Ser. No. 78,908, filing date Sept. 25, 1979, now abandoned, discloses a digital time multiplex communications network operating simultaneously for a plurality of different kinds of user instruments and having a decentralized exchange and a plurality of user, or subscriber, stations, with each user station being associated with at least one user instrument of at least one kind of user instruments, wherein the improvement comprises: separate signal conducting lines in each user, or subscriber, station for the conduction of signals in the transmitted and received directions, respectively, of the respective station; and autonomous units including identical channel occupation check units each connected in a respective station, and identical groups each composed of a connection control member and a format converter, with each such group being connected to a respective user instrument and to the channel occupation check unit, respectively, in that user station.
In time division multiplex systems with decentralized exchange, the subscriber stations are of particular importance from an engineering point of view and thus also from an operational and economical point of view. They must perform completely identical functions with the highest reliability at certain points in the network which have a high user density, which means at very many points of the network. Consequently, a considerable amount of the total installation costs falls onto these decentralized subscriber station exchanges.
It is well known from conventional centralized switching networks, e.g. from extensions, especially PABX, that call transfer, diversion of an existing connection or a connection which is to be built up, respectively, is possible and often done. Competitiveness of systems with decentralized switching is increased by offering such well-known features to the user in the same or even more comfortable manner.
An important step was taken in this direction by the Hasler AG, Bern (Swisse) with their Laboratory system for integrated local communication. The "Report 148 of the Int. Switching Symp. - ISS'74- Munich, Sept. 9 to 13., 1974" shows that in this system consisting of a closed ring main, or loop with decentralized exchange, such features as call back, call indication and call transfer are included. In the beginning of this report--page 148/1, right column, second paragraph--there is a statement, that each subscriber station is able to build up and terminate a connection without any cooperation of a central controling unit. This includes the features described above. There is, however, a more detailed description concerning call transfer--pages 148/2 and /3, last paragraph of part 3-- explaining that this feature is realized only for external calls and requires additional functions at an interface station. In order to materialize this the interface station has got to be equipped with three address registers, (S-sending address register; R--receiving address register; A--auxiliary/additional address register) like the subscriber station in the main ring. Two of them, the S-register and the R-register, are needed for simple connections. During such a connection the subscribers in the ring main exchange information as blocks of data, including the respective addresses. The A-registers serve to execute the features requested and store the contents of the R- and S-registers, respectively, by pressing one of the buttons (Asterisk) or red, and # (rhomb) or green.